This invention concerns the fabrication of copper-clad laminates useful in the production of printed circuit boards and, more particularly, it concerns itself with a novel method for producing such laminates and improved laminate products, as well as novel intermediate products.
Copper-clad laminate is one of the starting materials used in the production of printed circuit boards. Such a laminate consists of a substrate having a foil of copper firmly adhered thereto. Producers of printed circuit board (PCB) products apply the desired circuit patterns in different ways. The most common method, known as subtractive processing, involves masking the desired pattern by a photo-resist or screen printed masking material on the copper-clad laminate and then removing the undesired copper cladding by etching.
Another method for producing circuit patterns requires the use of a substrate clad with ultra-thin copper. Masking is applied as described above. However, the copper is exposed in the area in which the circuit pattern is desired. Electro-deposition is then performed increasing the circuit line thickness after which the masking and thin background copper are removed by etching. This approach is known as the semi-additive method.
It is desirable, of course, to produce PCB's having the maximum number of circuit lines contained thereon. The more circuit lines, and consequently, the more components, that can be fit on to a single board, the more compact and economical the product becomes. One of the limiting factors, however, in the number of circuit lines that can be applied in a given amount of space is the degree of fineness with which such lines can be produced. Another limitation is the degree of precision with which the lines themselves and the spaces between them can be defined.
Those skilled in the art realize that it is desirable in light of the foregoing objectives and for other reasons to use relatively thin foils in the production of the basis laminate product which is to be used in the production of PCB's. With the subtractive process applied to thicker foils, there is greater wastage of copper when the background foil is etched away, as described above. Also, there is necessarily a certain amount of side etching of the circuit lines themselves, reducing the amount of current carrying material and altering the surface morphology of the circuit lines. Obviously, this gives rise to a further limitation in how closely the circuit lines can be spaced from one another. Where semi-additive processing of laminates clad with thin copper foil is used, these disadvantages are clearly minimized.
Foils for copper-clad laminates have been produced for the most part by electrodeposition up to the present time. This process has many advantages, including speed of production, economy and a very fully developed technology associated with it. There are, however, certain limitations inherent in the electrodeposition process when this technology is extended to the production of ultra-thin copper foils. For one thing, there is growing concern about its environmental impact. For another, it is very difficult to produce foils of less than 16 microns thickness which are free of pinholes. The pinholes appearing in thinner electrodeposited foils result in our opinion from the presence of impurities or defects at random locations on the surface of the electrode upon which deposition is occurring or as a result of entrapment of impurities inherent in the electrodeposition process. These impurities thus prevent electrodeposition at those locations creating pinholes which may close only when a certain thickness is achieved. Furthermore, should one produce a substantially pinhole-free ultra-thin film electrolytically, that film will inherently be of relative large grain size. With ultra-thin films, particularly those in the very thinnest ranges, the average depth of the grain boundaries begins to approximate the thickness of the films themselves. Since some organic impurities will generally be collected at points in those boundaries, there is a possible weakening of such films at those points.
Bonding of such ultra-thin films in whatever manner to suitable substrates as laminates suited for PCB production can be accomplished with consistent success in the manner disclosed and claimed in our co-pending patent application referenced above. We have discovered, however, that the desired adherence between such copper films and their substrates can also be established without the necessity for altering the nature of the as-deposited copper film surface. In particular, we have found that through the use of ultra-thin films of one or another of certain metals in combination with a suitable oxide ultra-thin film, the desired bonding result can be produced. Thus, an ultra-thin film of such metal is provided on the copper film and is over-coated with an ultra-thin oxide film which is maintained in contact with the substrate as lamination is accomplished.
Metals suitable for use in accordance with this invention are those which are mutually soluble with copper to some extent and under the conditions of processing according to this invention can form strongly adhering oxides. Either the metal or its oxide must also be adherent to the subsequently applied oxide layer. Zinc, aluminum, tin and chromium, for instance, meet these requirements, while iron, for one, does not. Furthermore, it is desirable that the above-mentioned metals form solid solutions with copper that are readily etchable by the same etchants normally used in copper removal from printed circuit boards.
We have found that when zinc is used for the copper-bonding purpose of this invention and the zinc film is established by sputtering, system conditions should be adjusted so as not to favor the formation of a brass surface.
Oxides that may be used with consistent success in the practice of this invention include silicon dioxide and aluminum oxide, i.e., Si0.sub.2 and Al.sub.2 O.sub.3. Further, they may be used interchangeably with the various metals stated above, but in any event should be vapor deposited for best results in carrying out this invention process.
By using the method of our above-referenced patent application to form ultra-thin copper films, one using the present invention can consistently produce copper-clad laminates of extremely smooth, virtually pinhole-free surfaces for subsequent electrodeposition of circuit lines of high integrity. In other words, the principal advantages of our invention disclosed in our said co-pending patent application are retained in full measure in the practices and the products of the present invention. Thus, in addition to the thin film or foil advantages noted above our present new process and products have important advantages over their prior art counterparts in thin sheet applications and, consequently, like the invention of our co-pending patent application, the invention of this case is not strictly limited to the production and use of laminates with ultra-thin films or foils.
Briefly described, the present invention involves in its method aspect the vapor deposition on a layer of copper up to 80 microns thick, preferably on an ultra-thin film of copper, of an ultra-thin film of metal such as zinc, aluminum, tin or chromium, and then the vapor deposition of an ultra-thin film of silica or aluminum oxide on the resulting metal film, and finally laminating the silica- or alumina-coated metal body with a substrate so as to create relatively strong adhesion between the body and the substrate. The copper layer or film can be provided by vapor deposition, rolling or by electrolytic technique, but if high-resolution printed circuit patterns are desired, the copper should be in the form of an ultra-thin film. In accordance with our preferred practice, zinc is deposited by sputtering on the copper layer or film, and then during the course of the substrate lamination step as the assembly is heated, the zinc will alloy with the copper to produce brass. Staining of the substrate by the brass as experienced in the prior art practice does not result, however, because the alumina or silica coating is an effective barrier to the migration of staining compounds into the substrate. Also, preferably, the thickness of the zinc, aluminum, tin or chromium applied to the copper film is of the order of only 1,200 to 7,000 Angstroms while the thickness of the alumina or silica film is of only 200 to 1,200 Angstroms, and is produced by sputtering.
Having observed that when zinc is sputtered to form an ultra-thin film in the presence of a small amount of moisture, the film has an irregular microscopic surface morphology characterized by the presence of fine whisker-like dendrites, and having also observed that in some instances the presence of such dendrites is associated with good to excellent substrate adherence in the ultimate laminate product, our preferred practice involves the use of a sputtering atmosphere promoting such dendrite growth in moderate degree. Thus, in our preferred practice we use moisture-containing argon for this purpose, producing it by mixing dry argon in about equal proportions with moist argon obtained by transpiring water vapor in a stream of argon at room temperature.
Likewise, briefly described, a laminate product of this invention comprises a carrier coated with a release agent, a layer of copper up to 80 microns thick on the release agent, a vapor deposited ultra-thin film of zinc, aluminum, tin or chromium on the copper layer and a vapor deposited ultra-thin layer of silica or alumina on the surface of the vapor deposited film. Preferably, the copper layer is in the form of a vapor deposited ultra-thin film. Further, it will be understood that this product has utility for various purposes including the production of copper-clad laminates for PCB production particularly when the copper film is ultra-thin so that high resolution patterns can be readily produced by semi-additive or subtractive methods. In the latter event, the carrier together with the release layer is stripped off the copper film surface after a substrate is bonded to the assembly. In the case of thicker copper film laminates and products like that of FIG. 6, conventional printed circuit boards can readily be made, particularly by subtractive techniques, as the surface of the copper body is exposed for such processing on removal of the carrier or when a carrier is not employed, as in making the FIG. 6 laminate.